Drawing the screen: SetScreenHeight

       Name: SetScreenHeight                                         [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Set the screen height variables to the specified height
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEATH calls SetScreenHeight
             * RES2 calls SetScreenHeight
             * SendSpaceViewToPPU calls SetScreenHeight
             * TT22 calls SetScreenHeight


 Arguments:

   A                    The y-coordinate of the centre of the screen (i.e. half
                        the screen height)


.SetScreenHeight

 STA halfScreenHeight   ; Store the half-screen height in halfScreenHeight

 ASL A                  ; Double the half-screen height in A to get the full
                        ; screen height, while setting the C flag to bit 7 of
                        ; the original argument
                        ;
                        ; This routine is only ever called with A set to either
                        ; 72 or 77, so the C flag is never set

 STA screenHeight       ; Store the full screen height in screenHeight

 SBC #0                 ; Set the value of Yx2M1 as follows:
 STA Yx2M1              ;
                        ;   * If the C flag is set: Yx2M1 = screenHeight
                        ;
                        ;   * If the C flag is clear: Yx2M1 = screenHeight - 1
                        ;
                        ; This routine is only ever called with A set to either
                        ; 72 or 77, so the C flag is never set, so we always set
                        ; Yx2M1 = screenHeight - 1

 RTS                    ; Return from the subroutine